This invention relates to a process for producing high-quality steel using an oxygen top-blowing procedure. In the present invention and for purposes of this application, high quality steel is defined as a steel which contains as little phosphorus, sulphur and non-metallic inclusions as is reasonably possible.
A known method for achieving the desired carbon content in a melt comprises the discontinuance of the decarburizing process step at the pre-selected time. However, a disadvantage or drawback of this method is that it does not permit the production of steels having both high carbon and low phosphorus contents. Nevertheless, if it is desired to obtain steels having low phosphorus content, a pre-treatment (dephosphorizing, desulphurizing, desilicizing) of the hot metal melt is necessary. Such pre-treatments, which have been described primarily by Japanese steelmakers in various printed documents, are performed outside of steelmaking converters, in stationary or transportable ladles; and generate high costs due in part, to the expensive apparatus which must be used. It will be appreciated that the inevitable temperature losses inherent such processes have to be either borne or compensated through heating devices thus leading to increased costs and further disadvantages. It should be noted that only by using the method described above wherein the desired carbon content is obtained through discontinuing the decarburization step, will low inclusion (phosphorus) contents steel also be obtained.
Another known method which allows low phosphorus contents in steel to be obtained at lower costs, involves decarburizing the steel down to low carbon-contents, eg. 0.05% C. However, with this method, the decarburized steel will contain relatively high amounts of oxygen, depending upon the conditions (for example, 500 to 1000 ppm, and even more that that), the removal of which through conventional means not only requires large amounts of expensive substances, but also generates undesired inclusions.